Roles of malonate donor on activity and stereoselectivity of Ziegler–Natta catalyzed propylene polymerization

Abstract

The role of malonate compounds as electron donors in the propylene polymerization using Ziegler–Natta (ZN) catalyst were investigated. Adsorption modes of a malonate compound on the MgCl2(110) surface were also studied. Calculations revealed that the chelate mode was preferred over other modes (mono, bridge, and zip). For the malonate donor, the energy of the transition state of the primary (1,2) insertion of the propylene to Ti–iBu bond is lower than that of the secondary (2,1) insertion and that of the primary (1,2)-si is lower than that of the primary (1,2)-re. Without the malonate donor, the transition state of the primary (1,2)-si and -re has the similar energy. This suggests that in the presence of the malonate donor the ZN catalyst shows both stereo (favoring si face) and regioselectivity (favoring primary (1,2)) for polypropylene polymerization in agreement with experiments. The steric repulsion between the methyl group of propylene and the alkyl moiety of the catalyst controls the regioselectivity while the steric interaction between the propylene and the electron donor dictates the stereoselectivity. The electron donor also stabilized the transition state structure by transferring electrons to Ti resulting in the reduction of the activation energy and hence the catalyst becomes more active.